To get a rough idea of how many BTC have been burned so far, see https://blockchain.info/address/1CounterpartyXXXXXXXXXXXXXXXUWLpVr. Note that this page may show some false positives. You cannot just send bitcoins to that address with any client and expect to have them be properly burned. For more instructions on how to burn, e.g. with a Blockchain.info wallet, see Read The Docs.